Genome Sequence and Genetic Diversity of European Ash Trees.

The genome sequence and genetic diversity of European ash (Fraxinus excelsior) trees reveals the species’ varying susceptibility to ash dieback. Woodlands and forests around the world are increasingly susceptible to the spread of pests and pathogens resulting from climate change and global trade. In particular, ash trees across Europe and North America are currently threatened by the fungal disease ash dieback and infestation by the emerald ash borer beetle, respectively. Against this background, Richard Buggs and colleagues report the first genome sequence of an ash tree, the European ash Fraxinus excelsior, and the re-sequencing of 37 F. excelsior trees from across Europe. They find a number of genetic variants associated with reduced susceptibility to disease, and use these for an assessment of the susceptibility of host populations in an area newly under threat from the pathogen. On the basis of transcriptomic markers, they predict that ash trees in the UK will prove to be less susceptible to ash dieback than ash trees in Denmark. Ash trees (genus Fraxinus, family Oleaceae) are widespread throughout the Northern Hemisphere, but are being devastated in Europe by the fungus Hymenoscyphus fraxineus, causing ash dieback, and in North America by the herbivorous beetle Agrilus planipennis1,2. Here we sequence the genome of a low-heterozygosity Fraxinus excelsior tree from Gloucestershire, UK, annotating 38,852 protein-coding genes of which 25% appear ash specific when compared with the genomes of ten other plant species. Analyses of paralogous genes suggest a whole-genome duplication shared with olive (Olea europaea, Oleaceae). We also re-sequence 37 F. excelsior trees from Europe, finding evidence for apparent long-term decline in effective population size. Using our reference sequence, we re-analyse association transcriptomic data3, yielding improved markers for reduced susceptibility to ash dieback. Surveys of these markers in British populations suggest that reduced susceptibility to ash dieback may be more widespread in Great Britain than in Denmark. We also present evidence that susceptibility of trees to H. fraxineus is associated with their iridoid glycoside levels. This rapid, integrated, multidisciplinary research response to an emerging health threat in a non-model organism opens the way for mitigation of the epidemic.